Quick diffrential equation question.

When a mint is spherical mint is sucked, a simple model gives the rate of decrease of its radius as inversely proportional to the square of the radius. Initially, the radius of a mint is 5mm and after 5 mins the radius is 4mm

Okey, my question is, does the differential equation for this start:
I wasn't at school when this was taught(=( ) and there's no examples in my book on inverse proportion.
Thank-you